The character that opposes the Protagonist - they represent the problem or force that the Protagonist must overcome in order to succeed.
The Antagonist can also be an abstraction or idea that is then embodied in the Contagonist. For example in Star Wars Darth Vader is not the antagonist, the Empire is - Vader is 'merely' an agent of the Antagonist.
In some genres the Antagonist defines the whole piece. For example in a thriller it's the nature and timing of the Antagonists course of action that completely defines the goals and journey of the Protagonist - think of the Bond films here, the motivation and character of the Protagonist barely changes from movie to movie, it's the evil plot of the Antagonist that makes it all work. Likewise the Antagonist can be a lot more charismatic than the Protagonist and is often (at least initially) sexier.
Horror films are also entirely defined by the Antagonist. Movies like "Jaws", "Alien", "The fog", "The Thing" and many more use the Antagonists name as the title for this reason.
